Electrical precipitator



Feb. 16, 1960 c. WASSERMAN ELECTRICAL PRECIPITA'IOR Nun Filed Dec. 7, 1953 I N V EN TOR. CHA 245s was-519M011.

United States Patent ELECTRICAL PRECIPITATOR Charles Wasserman, Baltimore, Md., assignor to Kop pers Company, Inc., a corporation of Delaware Application December 7, 1953, Serial No. 396,616

16 Claims. (Cl. 183-7) The present invention relates to voltage-regulating apparatus for electrical load devices and more particularly to a voltage-regulating apparatus designed for use with electrical precipitators.

In the operation of electrical precipitators, it has been found that the most efiicient performance is obtained when the voltage between the electrodes of the precipitator is maintained at a value substantially higher than that at which sparks start to jumb between the electrodes. In order to accomplish this, voltage regulating apparatus has been designed whereby precipitating voltage is raised and lowered, as necessary to maintain the rate of sparking or flashing within prescribed limits. In so designing this apparatus, it has been found necessary to provide circuits which delay downward adjustment of the voltage regulator, so that the voltage supply to the precipitator will not be undesirably corrected by any stray or minor sparking in the precipitator which does not effect optimum performance. Such a predetermined delay has a tendency to permit the precipitator to operate under sparking conditionswhich do effect optimum performance during at least a portion of the period of delay. Moreover, mechanical devices which are readily subject to Wear and are difficult to maintain in effective operation, have been used as part of a complex arrangement in order to obtain the desired result.

The present invention avoids these aforementioned disadvantages, providing a completely electrical circuit. that permits instantaneous downward voltage adjustment responsive to only that sparking which effects optimum precipitator performance.

One of the features of the present invention is to provide to the primary of a high voltage rectifier transformer of an electrical precipitator a continuously rising voltage, having a preset rate of rise, which is instantaneously reduced a preset amount when a voltagevalue is reached causing undesirable current'surge.

Still another feature of the present invention is to provide a voltage regulator wherein the rate of rise of voltage-by the regulator and the magnitude of voltage reduction per surge directly determines the surging rate.

Another feature of the present invention is to provide a voltage regulator that automatically selects an optimum voltage range for a particular gas-dust condition of an electrical precipitator by automatically controlling the upper and lower voltage limits across the primary of the precipitator transformer maintaining a preset surging rate in the precipitator.

Another feature of the present invention is toprovide a surge detector for an electrical precipitator which can operate either on positive half wave, negative half wave or the full wave, depending on the application of the high voltage rectifier to the precipitator.

Another feature of the present invention is to provid an amplifying system that permits voltage regulation of asupply circuit over a comparatively wide range.

Another feature of the present invention is to provide a voltage regulating circuit that eliminates expensive 2,925,142 Patented Feb. 16, 1960 ICC rise through the induction means over a preset voltagev range and to drop instantaneously a preselected amount in accordance with sensed responses in the line translated to the control means by the sensing means.

It will be obvious to one skilled in the art that various changes can be made in the arrangement, form, and type of the various elements disclosed without departing from the scope or spirit of the present invention.

Figure l of the drawings is a diagrammatic view of a circuit in accordance with the present invention for energizing an electrical precipitator.

Figure 2 of the drawings is a diagrammatic view of a portion of the circuit illustrating a modification which permits half wave operation.

Referring to Fig. l of the drawings, the automatic regulator -is illustrated as being applied to the controlof an electrical precipitator which is diagrammatically represented as comprising an extended surface collecting electrode 20, in the form of a vertically disposed,

22 depending axially into the tube.

discharge electrode 22 and deposited principally on the inner surface of the tube 20 from which the collected material is removed at selected intervals by rapping. It. is to be understood that other type precipitators known: in the art can be used in combination with the automatic: regulator hereinafter disclosed.

The discharge electrode 22 of the disclosed precipita' tor is energized through a rectifier 24 which can be one of several types. For example, a mechanical, selenium: or vacuum tube type rectifier can be used. The rectifier- 24 is connected to the high voltage secondary winding 26 of a step-up transformer 28. The primary winding 30 of this precipitator transformer 28 is energized from a single-phase supply line L --L of the main supply circuit through resistor M-4 and the adjustable resistanceunit 32, both of which units are connected in series: with the lines L L It is to be noted that the use of resistor M-4 is particularly advantageous when the regulator is used in conjunction with mechanical high volt-- age rectifiers, and is not necessarily required when the regulator is used with electronic type rectifiers. Paralleling the resistor M-4 is the alternating current. winding 1-2 of the saturable reactor M3. Also connected in series with the supply line L L is the current transformer T3.

For the purposes of a clearer understanding, the auto-- matic supply regulator can be considered as made up of three parts: an amplifying circuit 1, a sensing circuit II, T

and a control circuit IlI. These circuits are respectively indicated on Figure 1 of the drawings by the broken line enclosures.

The amplifying circuit I, comprising a self-saturating magnetic amplifier M-l, which includes associated winding 9--10 and.1112 and the rectifiers SM, when enthe supply line L -L of the main supply circuit. A direct current source a-b is connected through an aujustable resistor R48 with proper polarity to bias winding 78 of the magnetic amplifier M-l in order to provide means to vary the output of the ,r'n'agn etic amplifier. This direct current source is energized frern 'the supply line 1718 through the power transformer T-4 and the rectifier S4. A switch 19 is positioned series with this supply line. The bias winding-'k-a 'Iid adjustable resistor R-iS permits a setting of fiii'x conditions by varying the voltage applied to said winding 7 8 within the amplifier M-l for the optimum erformance of the control winding 6, described hereinaftenof the amplifier and further provides a means for manually adjusting the voltage supply to the precipitator'. The sensing circuit II, which senses current surges in supply line L1L2 distinguishing between normal and abnormal surges in the line'com prises a c rrent transformer T3 which is in Series with the line L L The rectifier 5-6 to resistor R-S. The D.C. voltage across parallel R-6 will charge condenser C5 connected in with R-6, through the resistor R5. l

Under normal operating conditions, a very lowvoltage appears across resistor R-5 and, therefore, no current flows through resistor R-4 which has an" appreciably smaller resistance than resistor R-5 andwhich is connected in series with cold cathode tube V-3, the tube V-3 rand resistor R-4 being connected in parallel with the resistor R-S. However, when a current surge takes place in line L -L this surge is transmitted through transformers T-3, T-2 and rectifier 8-6", showing up across R-S, since the condenser C5 .does not have sufiicient time to charge up, the time constant betwe'e'n'G-S and R-6 being so preselected as to permit this; When this surge is reflected across R-S, it isof sufficient magnit'ude to cause the tube V3, which can be of the cold cathode'type, to conduct and accordingly voltageiap pears across the resistor R-4, which, as aforementioned, V

has a lesser resistance glow tubeV-3.

Through this sensing circuit II, it is to be noted, current surges in line L -L sensed by the current transformer T 3 are translated into voltage surges across the resistor R 3, the length of the voltage surges being'governedby the constants R5, R 6, and C5 It is to be noted'further'that in the place of the full wave rectifier s d which. transmits surges on both positive and; negative portionscf re supply in line L L to theresistor'jReti (Figure 1), a half wave-rectifier S7,--as' disclosed in Figure' 2,: can be used in. the sensing circuit II whereby'only the positive or negative rectified surges can be applied't'o the: resistor R6.- e e Broadly; the control circuit III connects, thesensing than R-5 and is in series with the positioned across the line c-d and comprising resistors R43, R4, R-9, and R-16.

Voltage gd, shunted by filtering condenser (1-3 is applied to condenser C6 through resistor R-7 and ad justable resistor P-3. The series connected resistors R-7 and P-3 provide a comparatively long time constant causing the voltage across condenser 0-6 to build up gradually. The adjustable. resistor P- -3 permits a varying of this time constant if so desired. The gradiial build-up of voltage, appearing across condenser 0-6 is applied between the grid ii2faridjcathode, 50 circuit er tube vn through resistors R'3, R 10 and P-2. As the voltage increases across 6,the; grid 42 of vacuum tube V-l becomes more positive with respect to cathode 59, permitting more current to' flow through the tube V-I and consequently winding 5--6 of M-l. Paralleling the enemies-49 9f tube V-2 is connected across the resistor R4 of the surge sensing circuit Il. As a result,

- each time there isa surge across R-4, the tube V2 is causedto conduct and accordingly, there is a resulting decreaseof voltageac'ross condenser C6, the condenser (Pd-and resistor R-l limiting the voltage reduction across C '6;per. surge. 'I t isto be noted that further adjustment of the voltage reduction across C' 6 per surge can be ob; tained by setting the voltage hd applied to cathode {it} of'tube V4 by varying resistor R1 6. Although V Z' is'.disclosed'.as a triode tube, itis to? be understood that other type vacuum tube such as a tetrode' or pento'de tub'e cagbeusee. I I

The voltage across the condenser C-6 functions in a e:

' cbrd'anc'e with-the surge across the resistor R-of the circuit ii with the amplifying circuit 1- by linking the V controllwinding 5--6 of the amplifying circuit 1" withth'e resistor R-dof these'nsing circuit'IL The control circuit III, which energizes and controls the control winding 56 of the amplifying circuit I, comprises asecondary voltage supply circuit including a sup ply line: 1516 across which is connected-the 'fprirnary' off the power transformer T-l. A switch 32 isp'o'sitio'ned' in 'series' with thevsupply line. A direct current voltage which condenser is positioned across" the'line c;d

i v h pplied to a voltagedivider circuit also sensing 'circu'it through the grid 38 of the tube V'-2" par-' all ejl with the condenser C=6. This voltage is, in turn, applied to the control grid 42 of the high vacuum pentod'e" tube V 1. The biasing circuit for the control grid 42' of the tube"V'1 is also energized from the supply line 15 -16 through the" transformer T-l to the lines -f, a DLC. voltage "beingobtained across the lines e f through another secondary winding of't'ransforrner T4 and rectifiers-S-4 and S-5.- This DC. voltage" is passed througha' filter section'R- l la and the condenser C 2;

trot grid 42' also functions in accordance with the surge across, the -resi'stor"R4 of the sensing circuit through the grid 38-of the tube V-2. The anode 48' and grid 4-4,

and cathode"- 50' andsuppressor grid" 46 of -the tube V -l are energized across' the--li'nes i-d-tliroug'hthe resistor" R 14'in series ivith'thi's line and the aforementioned' com trlol winding-5 6 of thema ne'ne amplifier. The pain ityot theener'gy supplie'd from the lines id-to' the" control windings 5 6 is' so arrangedthat-a'n increasing cur rent results in an increase in theoutput of magnetic amplifier M 1 with which the control windings are assodated and accordingly an increase across the winding" 3' 4' of-the satur'able reactor'-M-3. This; in turn; re sults inanincrease'of voltageacross the primary of the precipitatoi' transformer 30; r

A milliammeter 521's provided in the cathode line of the-tube V-2 in order to indicate' the number of surges that take place and a voltmeter 54'isprovidedacros's the resistor M- -4 in' order to show' the operating range of vacuumitube's such as a triode or teti-ode tub'e'ca b used. It-" is further tofb'ej understoodthat for ther fielr's throughout thecircuit; various type's'ofrcctifiei'scan A resistor" be used, such as selenium, mechanical or vacuum type.

In operation, voltage is applied to the precipitator discharge electrode 22 through the precipitator step-up transformer 30 and rectifier 24 from a supply circuit through main supply line L L In an advantageous embodiment of the invention, a 440 volt, 6O cycle voltage can he applied to the line L L However, it is to be understood that various voltages can be used for various precipitator conditions and various resistances, rectifiers and tubes can be used in the disclosed circuit.

If the precipitator is running smoothly and not sparking, there is a steady current flowing through current transformer T4; of the sensing circuit II, a steady voltage on the condenser C-5 of the sensing circuit II and no current through resistance R4 of the sensing circuit II.

At the same time, the current through control winding 5-6 of the amplifying circuit I slowly increases as a result of the energization of the control circuit III from the supply line 1516. This is caused by a gradual rise in voltage across the condenser C-6 of the control circuit III, which, in turn, causes the tube V-l of the control circuit III to gradually increase in conduction. Since the tube V-1 is in series with the control winding 5-6 of the amplifier circuit I, there is an increase 'of current through winding 56. This causes an increase of current through the direct current winding 3-4 of the saturable reactor M-3. As the voltage to the precipitator rises, it will reach a value, dependent upon the dust condition of the gas flow, to cause sparking or flashover.

When this happens, there is a current surge in the supply line L -L which is reflected across current transformer T-3, amplified through input transformer T-2 of the sensing circuit II, and rectified by rectifier 8-6. The constants of the sensing circuit II are adjusted so that the difference in voltage between resistor R-6 and condenser C-S, which does not have time to charge up as a result of the surge, is of sufiicient magnitude to cause the glow tube V-3 to conduct with a resultant surge appearing across the resistor R-4 of the sensing circuit II. The time length of this surge depends upon the time length of the surge in the main supply line, and its magnitude is in excess of a predetermined value as selected through choice of resistors and adjustment of the resistor R-17 in the sensing circuit II.

Since the grid circuit of the triode tube V-2 in the control circuit III is connected across the resistor R4 of the sensing circuit II, each surge in this resistor causes the tube V-2 to conduct and accordingly there is a resultant decrease in the voltage across condenser C-6 of the control circuit II.

As was aforedescribed, the increase in voltage across condenser -6 effects an increase of voltage in main supply line L L through tube V-l, magnetic amplifier M-1 and saturable reactor M-3. Accordingly, a decrease in voltage across condenser C-6 will effect a decrease in the voltage across the main supply line. This voltage will decrease until the sparking fails to create a surge in R-4. The voltage then starts to rise again as the condenser C-6 commences to charge upwardly again, the charging of the condenser 0-6 being gradual as a result of the long time constant established through the variable series resistors in the control circuit II.

It is to be noted that in certain instances, the amplifying circuit I need not be included as part of the overall circuit, the control circuit being connected directly to the reactor M-3, and it is further to be noted that the saturable reactor M-3 and magnetic amplifier M-1 can be used in conjunction with the sensing circuit without the use of the control circuit, whereby manual regulation can be obtained through the amplifier, the control windings 6 of the amplifier being connected to S-6 of the sensing circuit in order to obtain an automatically stabilized manual regulator.

The invention claimed is:

l. A voltage regulating apparatus for an electric load device comprising a main supply line forsaid load device; a saturable reactor having a main winding and a direct current winding, said main winding being connected in series with said supply line; a magnetic amplifier having a bias winding and an adjustable resistance means associated therewith and being connected to said direct current winding to control flux conditions within said saturable reactor; a sensing means operatively connected with said supply line for obtaining discrete current-responses to current surges in said circuit created by raised voltages across said load device; and a control means connecting said sensing means and said magnetic amplifier for causing said voltage in said supply line to gradually rise through said amplifier and said reactor over a preset voltage range and to drop instantaneously a preselected amount in accordance with sensed current responses translated to said control means by said sensing means.

2. A voltage regulating apparatus for an electric load device comprising a main supply line for said load device;

' a variable induction means positioned in said supply line;

a current transformer positioned in said line; an input transformer having a primary winding connected across said current transformer and a secondary winding; a rectifier connected to said secondary winding; a first resistance unit and a condenser connected in parallel across said rectifier; a second higher resistaince unit connected in series between said first resistance unit and said condenser; a third resistance unit of lower resistance than said second resistance unit connected in series with a vacuum glow tube acress said second resistance unit, whereby surges detected by said current transformer cause said glow tube to conduct, thereby causing surges of predetermined magnitude to appear across said third resistance unit; and a control means connecting said third resistance unit and said induction means for causing the voltage in said supply line to gradually rise through said induction means over a preset voltage range and to drop instantaneously a preselected amount in accordance with surges appearing in said third resistance unit.

3. The circuit of claim 2 and an adjustable resistance unit connected across said primary winding to thereby vary the magnitude of the surges to which the third resistance unit responds.

4. A voltage regulating apparatus for an electric load device comprising a main supply line for said load device; a variable induction means positioned in said main supply line; a sensing means positioned in said supply line for developing discrete current responses to current surges detected in said line created by raised voltages across said load device; a second supply line, said surges being connected to said variable induction means; a vacuum tube positioned in said second supply line, and a variable preset control means connecting said tube and said sensing means for causing said tube to increase in current conductivity when no surges are detected by said sensing means thereby causing the volttage in said main supply line to gradually rise through said induction means over a preset voltage range and for causing said tube to drop instantaneously a preselected amount in current conductivity when surges are detected by said sensing means thereby causing the voltage across said means supply line to drop instantaneously a preselected amount.

5. The apparatus of claim 4 wherein said variable control means comprises a condenser positioned in a circuit across said second supply line adapted to control the conductance of said vacuum tube; an adjustable resistance unit positioned in said circuit providing a long time constant for said condenser whereby the voltage builds up in said condenser gradually at a prefixed rate and the current conductivity of said vacuum tube in said second supply line accordingly increases at a prefixed:

rate; and a second vacuum tube paralleling said co'iid'ense'r, the grid ofsaid second vacuum tube being connected torsaid sensing means whereby surges detected by. said sensing means causes said vacuum tube to operate instantaneously thereby resulting in an instantaneous voltage drop across said condenser and a current decrease across said first vacuum tube. V

6. A sensing device for sensing current surges in a supply line of predetermined magnitude comprising a current transformer positioned in said line; an input transformer having a primary Winding connected across said current transformer and a secondary winding; a rectifier connected to said second secondary winding; a first resistance unit and a condenser connected in parallel across said rectifier; a second higher resistance unit con nected in series between said first resistance unit and said condenser; a third resistance unit of lower resistance than said second resistance unit connected in series with a vacuum glow tube across said second resistance unit, whereby surges detected by said current transformer cause said glow tube to conduct causing surges of predetermined magnitude to appear across said third resistance unit.

7. The circuit of claim 6 including an adjustable resistance unit connected across said primary winding to vary the magnitude of surges to which the third resistance unit responds. 1

8. A current control circuit for a supply line adapted to increase current in said line at a preselected'rate and to drop said current instantaneously in accordance with sensed surges imposed on said line comprising: a first vacuum tube positioned in said line; biasing means for said first vacuum tube; a condenser positioned across said line for controlling said biasing means; an adjust able resistance unit connected in series with said con enser for providing a long time constant for said con denser so that the voltage across said condenser builds up gradually at a prefixed rate to increase the current conductivity of said vacuum tube in said supply line at a prefixed rate and accordingly the current output of said line; and a second vacuum tube paralleling said condenser and adapted to be fired by said sensed surges for causing an instantaneous voltage drop across said condenser and a decrease of current through said first vacuum tube and said line.

9. A voltage regulating apparatus for an electric load device comprising: variable induction means having. a main winding and a direct current winding; a supply linerfor said load and connected in series with said main winding; a magnetic amplifier connected to the direct current Winding and having control windings adapted so that an increasing current therein results in an increase in -cu'rrent through the direct current winding of. said variable induction means and hence'the main supply line; a sensing means for developing discrete direct current responses to current surges in the supply line, said surges being created by raised voltages across said load device; saidsensing means comprising a main circuit including first and second resistors and a capacitor so arranged that the direct current voltage across said first resistor will charge. the capacitor through said second resistor; and a bypass circuit including a resistance unit in series with a cold cathode glow tube connected across said second resistor and normally carrying no current except as current surges transmitted .to the maincircuit cause said glow tube to conduct thereby causing corresponding voltage dropsacross said bypass resistance unit;

and control means connected to the; control windings of the magnetic amplifierand comprisin'ga secondary voltage supply circuit including a vacuum. tube circuit,

said secondary supply circuit causing thevoltage inthc maid-supply line to rise graduallythrou'gh the control windings and the variable induction means over a preset voltage range, and said vacuum tube *cir'cuit causing the'voltage in the" has supply line to drop instantane ously a-preselected amount in response to voltage drops across said bypass resistance unit.

-10.. Therapparatus of claim 9 wherein the grid of a vacuum tube of said vacuum tube circuit is responsive to voltage drops across the bypass resistance unit of said sensing means whereby said vacuum tube circuit causes the voltage in the main supply line to drop instantaneously a preselected amount in response to surges detected by the sensing means.

11. A voltage regulating apparatus for an electric load device comprising a main supply line for said load device; a saturable reactor'having' a main winding in series with said supply line and a direct current winding connected to a control means; a sensing means positioned in said supply line for obtaining discrete current responses to current surges in said line created by raised voltages across said load device; and said control meansrconnected to said sensing means and said saturable reactor, said control means being connected to said direct winding of'said saturable reactorthrough a magneuc ampl1- fier and causing said voltage in said supply line to gradually rise through said induction means over a preset voltage range and to drop instantaneously a preselected amount responsive to said sensed current responses translated to said control means by said sensing means.

12. An electrical precipitator forrremoving dust and foreign particles from a gas comprising a d scharge electrode for charging said particles, a collecting electrode for collecting said charged particles, an electrical energy source, a saturable reactor having a controlled wmdmg for operatively connecting said discharge electrode and said source and a control winding for controlllng said controlled winding, means connected'with said control winding actuating the latter so that said controlled w nd ing raises the voltage level of electrical energy supplied to'said discharge electrode gradually whereby surges of current to the discharge electrode develop when a'voltage level is reached at which sparking between said electrodes occurs, means developing a response to said surges, and means operative by a predetermined response for operat: ing said control Winding actuating means to cause said controlled winding to lower the voltage level of energy supplied to said discharge electrode instantaneously.

13. An electrical precipitator for removing dust and foreign particles from a gas comprising a discharge elec trode for charging said particles, at collecting electrode for collecting said particles, an electrical energy supply source, a first variable induction means including control means connecting said source and said electrodes for gradually raising the voltage level of the electrical energy supplied to said electrodes from a predetermined level to a level at which sparking between said electrodes occurs,-and means responsive to a predetermined magnitude of sparking actuating said control means to reduce said voltage instantly to said predetermined level. -V

14-. An electrical precipitator ,for removing dustand foreign particles from a gas comprising a dischargeelectrode, for charging said particles, a collecting electrode for collecting said particles, an electrical energy source, variable induction means including control means connecting said source and said electrodes and normally operable for raising the voltage level of the electrical energy supplied to said electrodes floma low'er predetermined level at a predetermined rate to a substantially higher level where sparking tends to jump between said electrodes, and means operative in response to sparking of a predetermined magnitude operating said control means to instantaneously lower said voltage to said predetermined level.

15. In an electrical precipitator wherein one electrode charges the dust and foreign particles of a gas with electrical energy supplied from a supply source and another electrode collects said charged particles to remove the particles from the-gas, a control for automatically selecting an optimum voltage range for the electrical energy .9 supplied to said electrodes for any gas-particle condition comprising variable induction means and including control means for gradually raising the voltage level from a lower level to an upper limit, said upper limit manitesting itself by current surges from said source due to sparking between said electrodes, and means operatively connected to said variable induction means and said con trol means responsive to said surges of a predetermined magnitude for instantaneously lowering said voltage to said lower level.

16. An electrical precipitator for removing dust and foreign particles from a gas comprising a discharge electrode for charging said particles, a collecting electrode for collecting said particles, an electrical energy source, a saturable reactor having a main winding forming a circuit for operatively connecting said discharge electrode and said source and a direct current winding, a magnetic amplifier having a bias winding and an adjustable resistance means associated therewith and being connected to said direct current winding to control flux conditions within said saturable reactor, sensing means operatively connected with said source for obtaining discrete responses to current surges in said circuit, said surges being 10 created by sparking between said electrodes due to the raised voltages across said electrodes and control means connecting said sensing means and said magnetic amplifier for causing the voltage supplied to said discharge electrode to gradually raise through said amplifier and reactor within a preset voltage range and to drop instantaneously in accordance with sensed current responses translated to said control means by said sensing means.

References Cited in the file of this patent UNITED STATES PATENTS 2,084,870 Schmidt June 22, 1937 2,297,740 Brown Oct. 6, 1942 2,530,993 Roman Nov. 21, 1950 2,569,605 Hall Oct. 2, 1951 2,673,324 Burton et al Mar. 23, 1954 2,719,255 Behr et al. Sept. 27, 1955 2,765,436 Dornhoefer Oct. 2, 1956 2,798,571 Schaelchlin et al. July 9, 1957 FOREIGN PATENTS 371,859 Great Britain Apr. 22, 1932 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,925,142 February 16, 1960 Charles Wasserman It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 33, for "acress" read across line 52, after "line" insert said surges being lines 53 and 54;,

strike out said surges being"; column 7, line 13, after "said" strike out "second".

Signed and sealed this 2nd day of August 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT WATSON Attesting Officer Commissions of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,925, 142 February 16, 1960 Charles Wasserman It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 33, for "acress" read across line 52, after "line" insert said surges being lines 53 and 54,

strike out said surges being"; column 7, line 13 after "said" strike out "second".

Signed and sealed this 2nd day of August 1960.

(SEAL) Attes't:

KARL H. AXLINE ROBERT WATSON Attesting Olficer Commissione of Patents 

